Understanding a Wren's sea exercises (9)
I believe the answer is:
awareness
'understanding' is the definition.
(I've seen this before)
'a wren's sea exercises' is the wordplay.
'exercises' indicates an anagram (letters exercise or move around).
'wrens'+'sea'='wrenssea'
'wrenssea' is an anagram of 'wareness'.
'a'+'wareness'='AWARENESS'
(Other definitions for awareness that I've seen before include "understanding" , "Recognition" , "State of being conscious" , "Consciousness, knowingness" , "Knowledge or consciousness" .)